An integrated circuit (IC) includes core logic and input/output (IO) circuits. The core logic performs a desired function and requires a low voltage termed as core supply voltage. The IC communicates with other ICs or external devices (filters, sensors etc.) at high voltage range termed as the IO supply voltage. The IO circuits act as an interface and communicate data between the core logic and the external devices. The IO circuits are connected to the external devices through board traces or metal wires, called transmission lines. An IO circuit includes a driver that drives signals on a pad to interface with the external devices. A bidirectional IO circuit has a driver used for sending signals to the external devices (transmit mode) and a receiver for receiving signals from the external devices (receive mode).
The IO circuits need to generate output signals to drive the external devices with suitable voltage levels compatible with the specifications of the external devices to ensure that the data is accurately transferred to the external devices. For example, if an external device is designed to receive input signals at +5 Volts, then output signals need to be at a maximum voltage of +5V. The IO circuits are generally implemented using low voltage transistors. In general, using low voltage transistors provides benefits such as high throughput performance, reduced electrical power consumption, lower number of fabrication masks (leading to lower fabrication costs) and high density (number of integrated circuits in a unit area).
A slew rate is a maximum rate of change of a signal at any point in the IC. The output voltage slew rate of the IO circuit is determined by the following factors, but not limited to, strength of the driver, operating conditions of the IO circuit, the parasitic capacitors and the loading capacitors in the IO circuit. A typical IO circuit is a series of inverters, which drive one another. The output slew rate of an inverter is proportional to the input slew rate. Thus, for high performance ICs, IO circuits with high driver strength are required. This increases the slew rate and thereby the supply current variations which cause the supply drop thus impacting the performance of the IO circuit. The control of slew rate of I/O circuit output voltage is very important as it is a major contributor to noise on the supply and ground lines. A fast rate of change of output voltage also affects electromagnetic interference (“EMI”), as well as signal reflection while driving a long cable, creating signal integrity problems in high speed serial data communication. Accordingly, it is desirable to implement the IO circuit generating high voltage output signals using low voltage devices and having slew rate control mechanism.